1. Field of The Invention
The present invention relates generally to a multifocal contact lens, a mold for forming the same, and a method for producing the same. More specifically, the invention relates to a multifocal contact lens with a lens curve formed by alternately arranging a plurality of curved surfaces for far vision and a plurality of curved surface for near vision in the form of concentric zones, a die for forming the same, a method for manufacturing the die, and a method for producing the multifocal contact lens.
2. Description of The Prior Art
A multifocal contact lens, wherein a plurality of portions for far vision and a plurality of portions for near vision are altermately arranged in the form of concentric zones, has been proposed in, e.g., Japanese Patent Laid-Open No. 59-146020. When a user wears such a contact lens, the user can consciously choose one of far and near ranges which can be simultaneously viewed by the user. This contact lens is useful since the user can naturally and smoothly choose one of the far and near ranges.
As shown in FIG. 8, such a contact lens 1 has a front curve 2 formed by altermately arranging a plurality of curved surfaces F1, F2, . . . for far vision and a plurality of curved surfaces N1, N2, . . . for near vision in the form of concentric zones, and a base curve 3 having a shape corresponding to the curved surface of the user's cornea.
In conventional contact lenses, the curved surfaces F1, F2, . . . for far vision and the curved surfaces N1, N2, . . . for near vision are formed on the front curve 2 as follows.
It is assumed that the radius of curvature of the curved surface for far vision is R.sub.F and the radius of curvature of the curved surface for near vision is R.sub.N. First, a circle having a radius R.sub.F from a point P on an optical axis corresponding to Z-axis is described to derive an intersection point with the optical axis. It is assumed that this intersection point is a center O.sub.F1 of curvature of the curved surface F1 for far vision. Then, a circle having the radius R.sub.F is described about the center O.sub.F1 of curvature to derive an intersection point P.sub.F1 with a straight line l.sub.F1 which defines a predetermined zone width of the curved surface F1 for far vision and which is parallel to the optical axis. Then, a circle having the radius R.sub.N is described about the point P.sub.F1 to derive an intersection point with the optical axis. In is assumed that this intersection point is a center O.sub.N1 of curvature of the curved surface N1 for near vision. Then, a circle having the radius R.sub.N is described about the center O.sub.N1 of curvature to derive an intersection point P.sub.N1 with a straight line l.sub.N1 defining a predetermined zone width of the curved surface N1 for near vision.
Similarly, a circle having the radius R.sub.F is described about the point P.sub.N1 to derive an intersection point with the optical axis. It is assumed that this intersection point is a center O.sub.F2 of curvature of the curved surface F2 for far vision. Then, a circle having the radius R.sub.F is described about the center O.sub.F2 of curvature to derive an intersection point P.sub.F2 with a straight line l.sub.F2 which defines a predetermined zone width of the curved surface F2 for far vision and which is parallel to the optical axis. Then, a circle having the radius R.sub.N is described about the point P.sub.F2 to derive an intersection point with the optical axis. It is assumed that this intersection point is a center O.sub.N2 of curvature of the curved surface N2 for near vision.
The centers O.sub.F1, O.sub.F2, . . . of curvature of the curved surfaces F1, F2, . . . for far vision and the centers O.sub.N1, O.sub.N2, . . . of curvature of the curved surfaces N1, N2, . . . for near vision thus obtained are shown in FIG. 7.
As can be clearly seen from FIG. 7, the centers O.sub.F1, O.sub.F2, . . . of curvature of the curved surfaces F1, F2, . . . for far vision are distributed so as to be sequentially shifted in a direction of Z-axis, i.e., from the base curve 3 toward the front curve 2.
As a result, as shown in FIG. 6, rays, which are incident on the curved surfaces F1, F2, . . . for far vision and which are parallel to the optical axis, form images at the respective focal points F.sub.F1, F.sub.F2, . . . of the curved surfaces F1, F2, . . . for far vision, so that the image formation is not carried out at a point. Similarly, rays, which are parallel to rays being incident on the curved surfaces N1, N2, . . . for near vision, form images at the respective focal points F.sub.N1, F.sub.N2, . . . of the curved surfaces N1, N2, . . . for near vision, so that the image formation is not carried out at a point. That is, there is a problem in that the spherical aberration of the conventional contact lens is too great to obtain a clear image.
Furthermore, as shown in FIG. 6, since the spherical aberration of the periphery of the contact lens 1 is great, the order of arrangement of the focal points F.sub.F1, F.sub.F2, . . . for far vision is the reverse of the order of arrangement of the centers O.sub.F1, O.sub.F2, . . . of curvature of the curved surfaces F1, F2, . . . for far vision.
In order to eliminate such a problem, a multifocal contact lens has been proposed by the applicant of the instant application as shown in FIG. 5 (e.g., Japanese Patent Application No. 5-508019 (International Publication No. 93/14434)).
As shown in FIG. 5, a contact lens 1 has a front curve 2 formed by altermately arranging curved surfaces F1, F2, . . . for far vision and curved surfaces N1, N2, . . . for near vision in the form of concentric zones, and a base curve 3. It is assumed herein that the optical axis of the contact lens 1 is Z-axis and the direction of the Z-axis is from the front curve 2 toward the base curve 3. It is also assumed that X-axis passes through the vertex P of the contact lens 1 and is perpendicular to the Z-axis.
The shape of the curved surface of the base curve 3 is chosen so as to correspond to the shape of the curved surface of the user's cornea. On the basis of the values for the chosen shape of the curved surface of the base curve 3, the radius R.sub.F of curvature of the curved surfaces F1, F2, . . . for far vision and the radius R.sub.N of curvature of the curved surfaces N1, N2, . . . for near vision, which are required to obtain desired powers of the portions for far vision and desired added powers of the portions for near vision, are defined.
The centers O.sub.F1, O.sub.F2, . . . and O.sub.N1, O.sub.N2, . . . of curvature of the curved surfaces F1, F2, . . . for far vision of the front curve 2 and the curved surface N1, N2, . . . for near vision of the front curve 2 are derived as follows.
As shown in FIG. 4, an intersection point of parallel rays being incident on the curved surface F1 for far vision and outgoing the base curve 3, with the optical axis is derived, and this intersection point is defined as a focal point F.sub.F for far vision. In addition, an intersection point of parallel rays being incident on the curved surface N1 for near vision and outgoing the base curve 3, with the optical axis is derived, and this intersection point is defined as a focal point F.sub.N for near vision.
First, it is assumed that the position on the optic axis, which is apart from the vertex P by the radius R.sub.F of curvature of the portion for far vision, is the center O.sub.F1 of curvature of the curved surface F1 for far vision. Then, a circle having the radius R.sub.F is described about the center O.sub.F1 of curvature to derive an intersection point P.sub.F1 with a straight line l.sub.F1, which defines a predetermined zone width of the curved surface F1 for far vision and which is parallel to the optical axis. Then, a circle having the radius R.sub.N is described about the point P.sub.F1 to derive an intersectional point with the optical axis. It is assumed that this intersectional axis is the center O.sub.N1 of curvature of the curved surface N1 for near vision. These steps are the same as those of the conventional method shown in FIG. 8.
Then, as shown in FIG. 5, the center O.sub.F2 of curvature of the curved surface F2 for far vision is derived as follows. That is, a circle having the radius R.sub.N is described about the center O.sub.N1 of curvature to derive an intersection point P.sub.N1 with a straight line l.sub.N1, which defines a predetermined zone width of the curved surface N1 for near vision and which is parallel to the optical axis. Then, a circle having the radius R.sub.F is described about the point P.sub.N1 to derive an intersection point with the optical axis. This intersection point is defined as a proposed point for the point O.sub.F2 which is to be the center of curvature of the curved surface F2 for far vision.
Then, the proposed point for the center O.sub.F2 of curvature is used as a start point to derive a rightful center O.sub.F2 of curvature near the proposed point using the ray tracing method. Specifically, the center O.sub.F2 of curvature is derived using the ray tracing method as follows.
First, a circle having the radius R.sub.F is described about the proposed point for the center O.sub.F2 of curvature to derive an intersection point of this circle with a straight line l.sub.F2. It is assumed that this intersection point is a proposed point for the point P.sub.F2 and that the curved surface extending from the point P.sub.N1 to the point P.sub.F2 is a proposed curved surface for the curved surface F2 for far vision.
Then, parallel rays are incident on the proposed curved surface for the curved surface F2 for far vision in the zone between the straight lines l.sub.N1 and l.sub.F2 to derive an intersection of the parallel rays outgoing the base curve 3 with the optical axis.
In a case where the intersection of the incident parallel rays with the optical axis is shifted from the focal point F.sub.F for far vision defined in FIG. 4 in a negative direction of Z-axis, the proposed curved surface for the curved surface F2 for far vision is rotated slightly counterclockwise about the point P.sub.N1 in FIG. 5.
On the other hand, in a case where the intersection of the incident parallel rays with the optical axis is shifted from the focal point F.sub.F for far vision in a positive direction of Z-axis, the proposed curved surface for the curved surface F2 for far vision is rotated clockwise about the point P.sub.N1.
Thus, the rotating direction and amount of the proposed curved surface for the curved surface F2 for far vision rotated about the point P.sub.N1 are derived and the rotating position is decided so that the parallel rays being incident on the front curve 2 pass through the focal point F.sub.F for far vision. The curved surface, which is obtained by rotating the proposed curved surface for the curved surface F2 for far vision about the point P.sub.N1 to the decided rotating position, is a curved surface F2 for far vision to be derived, and the decided center of curvature of the curved surface F2 for far vision is a center O.sub.F2 of curvature to be derived.
Then, a center O.sub.N2 of curvature of the curved surface N2 for near vision is derived as follows. That is, a circle having the radius R.sub.F is described about the decided center O.sub.F2 of curvature to derive an intersection point with a straight line l.sub.F2, which defines a predetermined zone width of the curved surface F2 for far vision and which is parallel to the optical axis, and this intersection point is defined as a point P.sub.F2. Then, a circle having the radius R.sub.N is described about the point P.sub.F2 to derive an intersection point with the optical axis, and this intersection point is assumed as a proposed point for the center O.sub.N2 of curvature of the curved surface N2 for near vision.
Then, the proposed point for the center O.sub.N2 of curvature is used as a start point to derive a rightful center O.sub.N2 of curvature near the proposed point using the ray tracing method. In order to derive the rightful center O.sub.N2, a circle having the radius R.sub.N is described about the proposed point for the center O.sub.N2 of curvature to derive an intersection point of this circle with the straight line l.sub.N2. This intersection point P.sub.N2 is used as a proposed point to define the shape of a proposed curved surface for the curved surface N2 for near vision extending from the point P.sub.F2 to the point P.sub.N2.
Then, parallel rays in the range between the straight lines l.sub.F2 and l.sub.N2 are incident on the proposed curved surface for the curved surface for near vision to derive intersections of the parallel rays with the optical axis after outgoing the base curve 3.
In a case where the intersections of the incident parallel rays with the optical axis are shifted from the focal point F.sub.N for near vision shown in FIG. 4 in a negative direction of Z-axis, the proposed curved surface for the curved surface N2 for near vision is rotated slightly counterclockwise about the point P.sub.F2 in FIG. 5.
On the other hand, in a case where the intersections of the incident parallel rays with the optical axis are shifted from the focal point F.sub.N for near vision in a positive direction of Z-axis, the proposed curved surface for the curved surface N2 for near vision is rotated clockwise about the point P.sub.F2.
Thus, the direction and amount of the proposed curved surface for the curved surface N2 for near vision rotating about the point P.sub.F2 is derived so that the parallel rays being incident on the front curve 2 pass through the focal point F.sub.N for near vision, and the rotating position is decided. The curved surface, which is obtained by rotating the proposed curved surface for the curved surface N2 for near vision about the point P.sub.F2 to the decided rotating position, is a curved surface N2 for near vision to be derived, and the center of curvature of the decided curved surface N2 for near vision is a center O.sub.N2 of curvature to be derived.
Similarly, other centers O.sub.F3, O.sub.F4, . . . and O.sub.N3, O.sub.N4, . . . of curvature are derived, so that the contact lens 1 shown in FIG. 5 is obtained.
However, in the case of the contact lens shown in FIG. 5, the spherical aberration may be sufficiently corrected for the following reasons.
That is, as shown in FIG. 5, although the center O.sub.F1 of curvature of the curved surface F1 for far vision and the center O.sub.N1 of curvature of the curved surface N1 for near vision are located on the optical axis, the centers O.sub.F2, O.sub.F3, . . . of curvature of the curved surfaces F2, F3, . . . for far vision and the centers O.sub.N2, O.sub.N3, . . . of curvature of the curved surface N2, N3, . . . for near vision are not located on the optical axis to be apart from the optical axis in a direction of X-axis. In addition, the ray tracing method is applied under the restriction that the curved surfaces F1, F2, F3, . . . for far vision must have the same radius R.sub.N of curvature and the curved surface N1, N2, N3, . . . for near vision must have the same radius R.sub.N of curvature When the ray tracing method is applied, the degree of freedom of design is decreased, so that the spherical aberration may remain as follows.
For example, with respect to the whole predetermined zone widths of the curved surface F2 for far vision and the curved surface N2 for near vision, there is considered parallel rays being incident in parallel to the optical axis and outgoing the base curve 3 to travel to the intersection with the optical axis. As shown in FIG. 4, these parallel rays converge before the optical axis after outgoing the base curve 3, and then, they are divergent rays. Therefore, no image is formed at a point on the optical axis defined by the line extending from the vertex P of the contact lens to the center O.sub.F1 of curvature of the curved surface F1 for far vision, so that spherical aberrations remain in the respective spherical surfaces of the curved surface F2 for far vision and the curved surface N2 for near vision.
More specifically, the parallel rays being incident on the curved surface F2 for far vision at a predetermined zone width, outgo the base curve 3 to converge at the respective points on a ring, which is described about the Z-axis so as to have a radius corresponding to the distance between the Z-axis and the point O.sub.F2. Thereafter, the rays become divergent rays, so that an image is formed on the Z-axis in the form of a line, not a point. In addition, the parallel rays being incident on the curved surface N2 for near vision at a predetermined zone width, outgo the base curve 3 to converge at the respective points on a ring, which is described about the Z-axis so as to have a radius corresponding to the distance between the Z-axis and the point O.sub.N2. Thereafter, the rays become divergent rays, so that an image is formed on the Z-axis in the form of a line, not a point. Thus, the parallel rays being incident on the respective curved surface do not form images at the focal point F.sub.F for far vision and the focal point F.sub.N for near vision on the curved surface defined above, so that spherical aberrations remain in the respective curved surfaces. This is the same with respect to the curved surfaces F2, F3, . . . for far vision and the curved surfaces N2, N3, . . . for near vision determined by the ray tracing method.
In addition, the dimension of the spherical aberration is under the influence of the absolute quantity of variation of the respective centers O.sub.F2, O.sub.F3, . . . and O.sub.N2, O.sub.N3, . . . of curvature in a direction of X-axis when the shapes of the curved surfaces F2, F3, . . . for far vision and the curved surfaces N2, N3, . . . for near vision are determined by the ray tracing method.
More specifically, when the shapes of the curved surfaces F2, F3, . . . for far vision and the curved surfaces N2, N3, . . . for near vision are decided by the ray tracing method, as the distances between the respective centers O.sub.F2, O.sub.F3, . . . and O.sub.N2, O.sub.N3, . . . of curvature and the optical axis in a direction of the X-axis is increased, the spherical aberration is increased.
Thus, in the case of the contact lens shown in FIG. 5, although the center O.sub.F1 of curvature and the center O.sub.N1 of curvature are located on the optical axis, the centers O.sub.F2, O.sub.F3, . . . of curvature of the other curved surfaces F2, F3, . . . for far vision and the centers O.sub.N2, O.sub.N3, . . . of curvature of the other curved surfaces N2, N3, . . . for near vision are not located on the optical axis. In addition, the ray tracing method is applied under the restriction that the curved surfaces F1, F2, F3, . . . for far vision have the same radius R.sub.F of curvature and the curved surfaces N1, N2, N3, . . . for near vision have the same radius R.sub.N of curvature. Therefore, there are problems in that the degree of freedom of design is not sufficient and that spherical aberration may remain.